The use of rare earth compounds as components of organo-metallic mixed catalyst systems for the stereospecific polymerization of 1,3-butadiene to afford a polybutadiene with a high cis-1,4-structure has been known for a long time. For example, U.S. Pat. No. 4,260,707 discloses a catalyst that may be used for polymerizing diene monomers in solution comprises (a) a reaction mixture formed by reacting a rare earth carboxylate and an aluminum compound wherein there are three hydrocarbon residues having from 1 to 20 carbon atoms attached to the aluminum, (b) a trialkyl aluminum and/or a dialkyl aluminum hydride and (c) a Lewis acid.
An improved process for preparing conjugated diolefin polymers or copolymers having a high content of 1,4-cis-units and high chain linearity is disclosed in U.S. Pat. No. 4,444,903. In this process a catalytic system prepared from (a) at least one carboxylate or alcoholate of a rare earth element, (b) a tertiary organic halide and (c) an organo metallic aluminum compound not containing halide ions of the formula: R.sup.a R.sup.b AlR.sup.c in which R.sup.a and R.sup.b are alkyl residues and R.sup.c is hydrogen or an alkyl radical.
Another example of the use of an organometallic mixed catalyst system containing a rare earth compound for producing a conjugated diene polymer is disclosed in U.S. Pat. No. 4,461,883. The process is characterized by polymerizing at least one conjugated diene with a catalyst consisting of (A) a reaction product of a Lewis base and a carboxylate of a rare earth element of the lanthanum series represented by AlR.sup.2 R.sup.3 R.sup.4 wherein R.sup.2,R.sup.3, and R.sup.4 which may be identical or different represent hydrogen or alkyl substituents, although R.sup.2,R.sup.3, and R.sup.4 may not all be hydrogen at the same time and (C) an alkyl aluminum halide represented by AlX.sub.n R.sup.5.sub.3-n wherein X is a halide, R.sup.5 is an alkyl group and n has the value of 1, 1.5, 2 or 3 and (D) optionally a conjugated diene.
U.S. Pat. No. 4,533,711 discloses that a polybutadiene rubber composition comprising a polybutadiene having a cis-1,4-content of at least 70 percent, an average chain length of 110 to 450 cis-1,4-units, a molecular weight distribution of at least 5.0 and containing at least 1 per cent by weight of a component having a molecular weight of not less than 25,000,000 is produced by polymerizing 1,3-butadiene in the presence of a catalyst system containing the compounds I to III as essential ingredients; (I) a compound of a lanthanum series rare earth element; (II) a trialkyl aluminum compound; and (III) a halogenated aluminum compound represented by a general formula of AlX.sub.n R.sub.3-n wherein X is a halogen atom, R is a hydrocarbon residue and n is 1 to 3; until the degree of conversion of 1,3-butadiene reaches at least 3 percent and then adding at least one compound selected from organo aluminum hydrides represented by a general formula of AlH.sub.m R.sub.3-m wherein R is a hydrocarbon residue and m is 1 or 2 and aromatic hydrocarbons containing active hydrogen as a modifier.
However, the foregoing catalyst systems are not without their disadvantages in that some of the catalyst systems are very difficult to dissolve in the hydrocarbon solvents which are commonly employed as the reaction solvent for polymerization of 1,3-butadiene. Moreover, the catalytic activity of some of the catalyst systems is not particularly high which is a disadvantage in industrial use.